THREE ARTERIAL COATS. 239 



contrary, collapse when cut across, unless that effect is pre- 

 vented by connections with surrounding parts. 



The arteries are composed of coats. In most parts of the 

 body they are enclosed in a sheath formed of dense areolar 

 tissue, their proper outer coat being connected to the sheath 

 by filaments of the same tissue so loosely, however, that when 

 the artery is cut across its ends readily shrink, each in an 

 opposite direction, some way within the sheath. Sometimes 

 other parts besides an artery are enclosed in one common 

 sheath : for example, the common carotid artery, the internal 

 jugular vein, and pneumogastric nerve are contained within 

 one common sheath. The arteries within the cranium are 

 destitute of sheaths. Arteries in general have three coats 

 besides the sheath; and this, by common consent, is now 

 received as the proper statement, when microscopic observa- 

 tion is not referred to. The inner coat of the arteries is 

 formed of epithelium and elastic layers the latter consisting 

 of elastic tissue, under the two forms of fenestrated membrane 

 and longitudinal elastic network ; while these may either co- 

 exist in equal proportion, or the one or the other may largely 

 predominate. The middle coat consists of distinct fibres 

 arranged circularly around the vessel, yet not so as to form 

 complete rings. A great part of the thickness of the larger 

 arteries is made up of this coat, which may include many 

 layers. In the smaller arteries this coat is said to be thicker 

 in proportion to the calibre of the vessel. Between the layers 

 are found shreds of elastic membrane, either finely reticular 

 or wholly similar to the fenestrated membrane of the inner 

 coat. It does not appear that any part of the middle coat of 

 the arteries is identical with the yellow elastic tissue, as was 

 believed for a time by anatomists. It contains two kinds of 

 fibres : the one corresponding to the plain variety of mus- 

 cular fibres, and the other fine elastic fibres joined together in 



